Bring the beef to room temperature for at least 30 minutes and season with salt and pepper. Peel and chop the potatoes and garlic. Slice the leek into rounds and the mushrooms into wedges.

Boil the potatoes in salted water for 20–25 minutes, or until tender. Drain the water and add 1–2 tablespoons of butter to the same pot. Mash the potatoes with a fork or masher until smooth and season with salt.

Heat oil and butter in a pan and cook the beef on both sides for 3–5 minutes, until it feels pleasantly springy to the touch. Remove the meat from the pan and let it rest for 5–10 minutes. Slice before serving.

In the same unwashed pan, add the garlic, leek, and mushrooms, and sauté until they start to brown. Then add 600 ml of water, bring to a boil, and reduce the liquid by about half.

Remove the pan from the heat and stir in the sour cream. Season with herb mix, salt, and pepper.

Serve the beef with mashed potatoes and the mushroom-leek sauce.